This invention relates to charged particle optical system for a charged particle exposure apparatus, in particular a maskless lithography system using charged particles.
The development of lithography systems in particular is driven by Moore's law i.e. the number of transistors per unit area doubles every 18 months. Consequently the feature sizes decrease rapidly resulting in a sharp increase of the costs of a masks currently used for providing a pattern. To avoid the increasing mask costs several maskless lithography concepts are under development. In these concepts the pattern is represented by pattern data. Since a mask is a highly efficient way to store a pattern, the amount of raw data describing such a pattern is enormous.
Current maskless lithography systems are limited in throughput, i.e. the number of processed wafers per hour. This feature limits the use of these systems in present day semiconductor element processing lines. The throughput of a maskless lithography system can be enhanced by using a plurality of beamlets and/or by increasing the data rate.
The supply of data can be increased in two ways. A first way of increasing the data rate is by sending the pattern data directly to the beam source or sources, thus switching the source of sources on and off. Alternatively, the source or sources continuously emit one or more beamlets and the pattern data is provided to modulation means that modulate the emitted beamlets along their pathway towards the target to be patterned.
The data supply to the sources becomes a problem when the data rate increases: Each individual source has a settling time that is source-dependent and becomes too large easily. It is therefore preferred to modulate the beamlets along their optical pathway.
In charged particle beam lithography systems, these modulation means are often electrostatic deflection arrays, also known as blanking aperture arrays (BAA) or deflection arrays. Examples of such arrays are disclosed in U.S. Pat. No. 6,188,074 by Advantest and in EP-patent application 1253619 by Canon. Upon the supply of an electric signal towards a certain deflection element, an electric field is established over a corresponding aperture, which results in the deflection of a charged particle beam passing through the aperture. By positioning an aperture plate behind the deflection array, wherein the apertures are aligned with apertures in the deflection array, the deflected beams are blocked and therefore do not reach the target.
Before the charged particles reach the deflection array, the beams most often pass an aperture array. This aperture array has several functions. In lithography systems comprising a single source it is used to split an emitted beam in a plurality of beamlets. Furthermore, it determines the opening angle of the beam at the deflection array. Additionally the aperture array reduces the heat load on the deflection array, thereby enhancing its performance.
Especially in compact multibeam designs, misalignment of the consecutive components (lenses, apertures etc.) of the system or a slight change of the position of the beam by for instance external electromagnetic fields, for instance resulting from charging of surfaces or irregularities on charged surfaces, results in dose variations. As a result of the dose variation the control of the critical dimensions of the features to be patterned is no longer guaranteed. One way of dealing with misalignment problems is increasing the opening angle of each beamlet, i.e. making the cross section of a beamlet on an aperture array larger than the aperture which is passes. In that way, it is ensured that the entire area of an aperture is illuminated.
This approach has several drawbacks, which become pertinent when high speed and extreme dose stability are required. The relatively large cross section will increase the heat load on an aperture array. Furthermore, the amount of deflection needed to completely block a beamlet will increase (see drawings for an explanation), which will reduce the speed and thus throughput of such a system. Also, for accurate dose control at the position of a substrate it is desired to approach a tophat distribution of the beam intensity as much as possible. A large opening angle of a beam causes a beam to have large tails.